1. Field of the Invention
The present invention relates to an apparatus, a computer program, a central venous catheter assembly and a method for hemodynamic monitoring.
2. Description of the Related Art
One of the most relevant parameters in hemodynamic monitoring is cardiac output, which currently is usually measured using intermittent bolus thermodilution measurements as described e.g. in U.S. Pat. Nos. 3,651,318, 4,217,910 and 4,236,527.
For bolus thermodilution measurements a certain amount of a liquid at a temperature higher or lower than blood temperature is injected as a bolus through a catheter which is placed in the blood stream of a patient, and the temperature deviation of the blood as a function of time is monitored at a place downstream from the place where the bolus is injected. The Cardiac Output CO can then be determined by algorithms based on the Stewart-Hamilton-equation:       C    ⁢          xe2x80x83        ⁢    O    =                              V          L                ⁡                  (                                    T              B                        -                          T              L                                )                    ⁢              K        1            ⁢              K        2                    ∫              Δ        ⁢                  xe2x80x83                ⁢                              T            B                    ⁡                      (            t            )                          ⁢                  ⅆ          t                    
where TB is the initial blood temperature, TL is the bolus temperature, VL is the bolus volume, K1 and K2 are constants to consider the specific measurement setup, and xcex94TB(t) is the blood temperature deviation as a function of time with respect to the baseline blood temperature TB.
According to the prior art, usually pulmonary artery catheters are used injecting the bolus through a lumen outlet substantially apart from the distal end of the catheter and detecting the temperature deviation of the blood as a function of time at the distal end of the catheter.
In a typical measurement procedure, a cold bolus of saline at ice or room temperature in an amount of about 5-10 milliliters, or as a guideline 0.1 ml/kg body weight is injected through the pulmonary catheter. Completion of this procedure takes about 2 minutes. In order to obtain sufficient accuracy, it is repeated several times and the results are averaged. Using this conventional method, determining one reliable cardiac ouput value thus requires a measurement time of up to 10 minutes and involves the injection of up to 50 ml of fluid into the cardiovascular system of the patient and, as a consequence, can be carried out only one or two times per hour.
Instead of using cold bolus injections, heating pulmonary artery catheters are used for modified thermodilution methods such as described in U.S. Pat. Nos. 4,507,974 and 5,217,019. Periodic heat pulses at a given pattern are introduced substantially apart from the distal end of the catheter by a heating coil or a thermal filament mounted to the indwelling catheter. Temperature changes of the blood heated passing the heating coil or thermal filament, respectively, are measured downstream by a thermistor at the distal end of the catheter. Cardiac output is determined quasi-continuously based on the data sampled for several minutes using signal processing and averaging algorithms. Thus, the cardiac output values determined are average values over periods of several minutes. Accuracy of modified thermodilution methods using heating pulmonary catheters is sensible to high and unstable blood temerature.
The amount of heat which can be introduced the way described above is limited to avoid a thermocoagulation of the blood or damage to the tissue adjacent to the heater. Approaches to regulate the heat transferred by the catheter are described in U.S. Pat. Nos. 5,701,908 and 5,857,976.
Generally, the use of pulmonary artery catheters is rather highly invasive and includes risks for the monitored patient, such as malignant arrythmias and pulmonary artery rupture or infarction. The so called transpulmonary thermodilution method is less invasive applying a central venous catheter instead of a pulmonary artery catheter. A thermal indicator is injected through a lumen of the central venous catheter and an additional arterial catheter, introduced for example into the femoral artery, is used to detect the thermodilution curve after the thermal indicator has passed the heart. Again, completion of this procedure takes several minutes and is not suitable for repeated cardiac output determinations within short time intervals.
Pulse contour analysis is a continuous method to determine cardiac output by multiplying the heart rate by a beat-to-beat stroke volume for each heart beat. The stroke volume is determined from the shape of and the area under the arterial blood pressure curve of the systole, wherein the arterial blood pressure is measured in the femoral artery or other large artery. In the currently commercially available System PiCCO by Pulsion Medical Systems AG, Germany the so called pulse contour cardiac output PCCO is determined using the relation       P    ⁢          xe2x80x83        ⁢    C    ⁢          xe2x80x83        ⁢    C    ⁢          xe2x80x83        ⁢    O    ∝      H    ⁢          xe2x80x83        ⁢    R    ⁢                  ∫                  S          ⁢                      xe2x80x83                    ⁢          y          ⁢                      xe2x80x83                    ⁢          s          ⁢                      xe2x80x83                    ⁢          t          ⁢                      xe2x80x83                    ⁢          o          ⁢                      xe2x80x83                    ⁢          l          ⁢                      xe2x80x83                    ⁢          e                    ⁢                        (                                                    p                ⁡                                  (                  t                  )                                                            S                ⁢                                  xe2x80x83                                ⁢                V                ⁢                                  xe2x80x83                                ⁢                R                                      +                                          C                ⁡                                  (                  p                  )                                            ·                                                ⅆ                  p                                                  ⅆ                  t                                                              )                ⁢                  ⅆ          t                    
wherein HR is the heart rate, p(t) the time dependent driving blood pressure (i.e. the time dependent arterial blood pressure AP(t) measured minus the mean central venous blood pressure CVP), dp/dt the first derivative of p(t) with respect to time, SVR the systemic vascular resistance and C(p) the aortic compliance function of the monitored patient. The integral is determined for the systolic phase. Calibration is achieved by conventional cold bolus injection transpulmonary thermodilution methods. The compliance function C(p) characterizes the volume change and subsequent pressure change of the aorta due to the ability of the aorta to expand during the systole and recontract during the diastole. C(p) and SVR also have to be determined by using a reference thermodilution measurement. A method and an apparatus to determine the compliance function C(p) are described in DE 19814371 A1 enclosed herewith by reference. In order to achieve reliable results over an extended period of time, recalibration is necessary. According to the current state of the art, in clinical practice the time consuming recalibration procedure of carrying out a conventional cold bolus injection transpulmonary thermodilution measurement is usually performed every 8 hours.
It is therefore an objective of the present invention to provide means rendering possible continuous recalibration of pulse contour measurements with only short time intervals between two respective recalibration steps. It is also an objective of the present invention to increase accuracy in determining hemodynamic parameters and, at the same time, keep the invasiveness of applying the medical equipment necessary as low as possible.
In order to accomplish these objectives, the present invention provides an apparatus for hemodynamic monitoring, which comprises a central venous catheter assembly comprising at least one lumen and heating means in the immediate proximity of the distal end of the central venous catheter assembly. The heating means are adapted to emit heat pulses for introducing travelling temperature deviations to a patient""s blood circulation. The apparatus further comprises means for determining the power transferred by the heating means as a function of time. The power transferred during emission of each heat pulse thereby represents an input signal corresponding to the respective heat pulse. The apparatus further comprises an arterial catheter assembly comprising temperature sensing means for measuring the local blood temperature in an artery of the patient as a function of time, thus determining for each input signal a corresponding system response. The apparatus further comprises computing means having implemented thereon executable operations for determining a reference cardiac output value of the patient from at least one input signal and respective corresponding system response.
In a preferred embodiment of the invention the computing means has additionally implemented thereon executable operations for determining global end-diastolic volume and/or intrathoric blood volume and/or extravascular thermovolume of the patient from at least one input signal and respective corresponding system response.
In another preferred embodiment of the invention the central venous catheter assembly is adapted to emit the heat pulses at random or at at least one predefined pattern, such as sinusoidal waves or step functions with predefined amplitudes and durations.
In another preferred embodiment of the invention the heating means comprise an electrically heatable filament or heating coil.
In another preferred embodiment of the invention the heating means are means for emitting electromagnetic radiation comprising a fiberoptic bundle which is either insertable into a lumen of the catheter assembly or mounted into a lumen of the catheter assembly.
In another preferred embodiment of the invention the electromagnetic radiation is modulated electromagnetic radiation at different wavelenghts.
In another preferred embodiment of the invention the fiberoptic bundle comprises at least one fiber adapted to collect reflected radiation at the different wavelengths and the computing means additionally has implemented instructions thereon to determine from the reflected radiation the central venous oxygen saturation.
In another preferred embodiment of the invention, the central venous catheter assembly comprises means, e.g. a pressure sensor mounted into one of the lumina of the central venous catheter assembly, for measuring the central venous blood pressure. Preferably, the means are adapted to measure the central venous blood pressure continuously or intermittently.
In another preferred embodiment of the invention determining the reference cardiac output value includes statistic evaluation of a plurality of the input signals and respective corresponding system responses.
In another preferred embodiment of the invention the arterial catheter assembly additionally comprises pressure sensing means for measuring the patient""s arterial blood pressure AP(t) as a function of time, thus creating an arterial blood pressure curve, and the computing means has implemented thereon executable operations for repeatedly determining hemodynamic parameters from the arterial blood pressure curve by pulse contour analysis using the reference cardiac output value.
In another preferred embodiment of the invention the computing means additionally has implemented thereon instructions to recalculate the hemodynamic parameters obtained from the arterial blood pressure curve and the reference cardiac output value after determining the reference cardiac output value.
In another preferred embodiment of the invention, the central venous catheter assembly comprises an additional lumen for thermal indicator injection. Thus a cold bolus transpulmonary thermodilution measurement can be used to obtain an initial reference cardiac output value for initial calibration.
In order to accomplish the above mentioned object, the present invention also provides a computer program for hemodynamic monitoring, which comprises instructions executable by a computer system to repeatedly perform the steps of
(a) determining a reference cardiac output value by statistic evaluation of a plurality of input signals and respective corresponding system responses, the input signals representing the power transferred during emission of a plurality of heat pulses as a function of time, the heat pulses introducing travelling temperature deviations to a patient""s blood circulation, the corresponding system responses being determined by measuring the local blood temperature in an artery of the patient as a function of time, and
(b) determining hemodynamic parameters from an arterial blood pressure curve by pulse contour analysis using the reference cardiac output value, the arterial blood pressure curve being determined by measuring the arterial blood pressure in an artery of the patient as a function of time.
In order to accomplish the above mentioned object, the present invention also provides a hemodynamic monitoring method comprising the steps of
(a) providing a central venous catheter assembly having a proximal end and a distal end, the central venous catheter assembly comprising at least one lumen and heating means in the immediate proximity of the distal end,
(b) emitting heat pulses through the heating means for introducing travelling temperature deviations to a patient""s blood circulation,
(c) determining the power transferred by the heating means as a function of time, the power transferred during emission of each heat pulse thereby representing an input signal corresponding to the respective heat pulse,
(d) providing an arterial catheter assembly comprising temperature sensing means and pressure sensing means
(e) measuring through the temperature sensing means the local blood temperature in an artery of the patient as a function of time, thus determining for each the input signals a corresponding system response,
(f) measuring through the pressure sensing means the patient""s arterial blood pressure AP(t) as a function of time, thus creating an arterial blood pressure curve, and
(g) determining a reference cardiac output value of the patient from at least one the input signal and the respective corresponding system response.
Hemodynamic parameters are repeatedly determined from the arterial blood pressure curve by pulse contour analysis using the reference cardiac output value.
In a preferred embodiment of the invention determining the hemodynamic parameters comprises the steps of
(1) calculating a mean arterial blood pressure MAP from the measured arterial blood pressure AP(t),
(2) calculating a systemic vascular resistance SVR of the patient according to the formula
SVR=(MAPxe2x88x92CVP)/COref
xe2x80x83wherein CVP is an arbitrary central venous pressure which is measured (preferably continuously or intermittently), ascertained or estimated, and COref is the reference cardiac output,
(3) calculating the driving blood pressure p(t) according to the formula
p(t)=AP(t)xe2x88x92CVP
(4) calculating at least the first derivative of the driving blood pressure with respect to time {dot over (p)}(t)=dp/dt, and
(5) determining the hemodynamic parameters from at least p(t), {dot over (p)}(t) and SVR according to a non-linear model.
In another preferred embodiment of the invention the step of determining the hemodynamic parameters from at least p(t), {dot over (p)}(t) and SVR according to a non-linear model includes determining a blood flow q(t) on the basis of the driving blood pressure p(t) and the first derivative with respect to time {dot over (p)}(t), and calculating a compliance function according to       C    ⁡          (      p      )        =                    q        ⁡                  (          t          )                    -                                    p            ⁡                          (              t              )                                /          S                ⁢                  xe2x80x83                ⁢        V        ⁢                  xe2x80x83                ⁢        R                                      p          .                ⁡                  (          t          )                    -                        Z          ⁡                      (            p            )                          ·                  (                                                    q                .                            ⁡                              (                t                )                                      -                                                                                p                    .                                    ⁡                                      (                    t                    )                                                  /                S                            ⁢                              xe2x80x83                            ⁢              V              ⁢                              xe2x80x83                            ⁢              R                                )                    
for arbitrary impedance functions Z(p) and arbitrary times t in such a way that       q    ⁡          (      t      )        =                    p        ⁡                  (          t          )                            S        ⁢                  xe2x80x83                ⁢        V        ⁢                  xe2x80x83                ⁢        R              +                  C        ⁡                  (          p          )                    [                                    p            .                    ⁡                      (            t            )                          -                              Z            ⁡                          (              p              )                                ·                      (                                                            q                  .                                ⁡                                  (                  t                  )                                            -                                                                    p                    .                                    ⁡                                      (                    t                    )                                                                    S                  ⁢                                      xe2x80x83                                    ⁢                  V                  ⁢                                      xe2x80x83                                    ⁢                  R                                                      ]                              
is optimally satisfied.
In another preferred embodiment of the invention the hemodynamic parameters comprise pulse contour cardiac output PCCO and/or stroke volume SV and/or stroke volume variation SVV.
In order to accomplish the above mentioned object, the present invention also provides a central venous catheter assembly for hemodynamic monitoring, which comprises at least one lumen and heating means in the immediate proximity of the distal end.
In a preferred embodiment of the invention the lumen into which a fiberoptic bundle for emitting electromagnetic radiation for introducing travelling temperature deviations to the patient""s blood circulation is insertable or mounted has an opening closer to the distal end of the catheter assembly than all the openings of all the other lumina of the catheter assembly.